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BAGHDAD, Iraq (CNN) -- Private military contractors are earning billions of dollars in Iraq -- much of it from U.S. taxpayers.

Business is booming for those willing to tackle one of the most dangerous jobs on Earth. Lucrative U.S. government contracts go to firms called on to provide security for projects and personnel -- jobs that in previous conflicts have been done by the military.

A single contract awarded to Britain's AEGIS Specialist Risk Management company by the Pentagon was worth $293 million, and while the government says it cannot provide a total amount for the contracts -- many of which are secret -- industry experts estimate Iraq's security business costs tens of billions of dollars.

These contractors have not been without controversy. Late last year, AEGIS launched an investigation into whether its employees produced video clips that showed up on the Internet in which it appeared civilian vehicles were being shot at. AEGIS has not released the results of its investigation, but a U.S. Army investigation found no probable cause that a crime occurred. (Watch the shootout that started the fracas -- 8:18)

The market for private contractors is there thanks to an unprecedented "outsourcing" of conflict, according to Amy Clark, who led the Baghdad end of a small private security contractor.

"Where you've got a military where the assets and the personnel are strained, then private contractors have had to step in and fill the void," she told CNN, agreeing to be interviewed if her company's name was not revealed.

But where there is money, there is also danger. No official totals exist of how many private contractors have been killed in Iraq. But Clark believes the death rate among the 25,000 or so contractors is higher than among U.S. military forces.

Going where the military won't

The danger does not bring glamour. Clark's outfit shepherds convoys along supply lines strewn with roadside bombs targeting U.S. and Iraqi forces and those who support them. Missions have included guarding trucks carrying gravel for military bases.

"Military doesn't even like to go where we are going, and most of the companies that do this don't want to go where we are going ... and that's why we're going," explained one of Clark's men, nicknamed "Mr. GQ."

His colleague, Gonzo, gives a graphic description of what their team faces: "If we get ambushed and cut off, then yes, we are going to fight back and push through. That's what we get paid to do -- protect the clients, protect the asset -- that's our job. (Watch Gonzo get locked and loaded to guard gravel -- 6:12)

"It sounds crude, but basically our job is to be a bullet sponge."

There is debate about how far these private contractors should go, what authority they have and who should police them, and no hard and fast answers. In the meantime, the contractors continue to face danger.

On one day recently, two roadside bombs went off simultaneously near one of Clark's security trucks, and the convoy was then attacked with heavy small-arms fire from nearby rooftops.

"The blood in the back seat of the truck, all the bone fragments and flesh pretty much told the tale -- they got hit pretty bad," Gonzo said.

That same night, three roadside bombs were detonated beside the same convoy. Two of Clark's men were killed and five wounded.

A year's pay in 3 months

There is plenty of money and plenty of work to go around, much of it taken by Blackwater -- one of the larger companies and perhaps the best known, because tragedy befell its employees in Falluja March 31, 2004. Four employees were killed -- two of their bodies hung from a bridge.

Blackwater was founded in 1997, and business boomed after 9/11. Wartime demands are allowing it to expand even further, and it recently opened new headquarters in North Carolina, where it can train people from the military and law enforcement.

Blackwater also looks for opportunities beyond war zones to disaster areas, such as the Gulf Coast after Hurricane Katrina, or places where peacekeepers could be stationed, like the crisis-hit region of Darfur in Sudan.

Cofer Black, a former head of the CIA Counterterrorism Center and now vice-chairman of Blackwater, said the company is ready to tackle more hot spots.

"My company could deploy a reasonable small force under guidance or leadership of any national authority and do a terrific job of protecting, you know, innocent women from being raped, young kids from having their arms hacked off with machetes."

Like most contractors, Gonzo is ex-military and has specific personal reasons for being in Iraq and facing the danger.

A veteran of the first Gulf War, he says he can earn in three months what it would take him a year to get in the United States. "My wife and I are pretty frugal. My goal is pretty simple -- I just want to be able to pay off a house and some property."

He holds up a picture of his three children. "We all have to be over here for a reason. Mine's so that I can provide a better life for my wife and kids."